The invention described herein may be manufactured and used by or for the Government of the United States of America for government purposes without the payment of any royalties therefor.
The invention relates in general to optical measurement of pressure and in particular to optical measurement of pressure based on an optical signal generated by a crystal.
The measurement of transient pressures in a shock process is crucial in designs of explosive devices, weapon systems, mine detection and remediation, armors and many other applications. Some known methods use ruby fluorescence to measure the temporal profile of pressure as a shock traverses a material. Two groups have demonstrated ruby fluorescence as a dynamic pressure gauge (See P. D. Horn, and Y. M. Gupta, Phys. Rev. 39, 973 (1989) and G. I. Pangilinan, M. R. Baer, J. Namkung, P. Chambers, and T. P. Russell, Appl. Phys. Lett. 77, 684 (2000)). Apparatus for measuring pressure using ruby fluorescence are disclosed in U.S. Pat. No. 5,293,046 to Wheatley; U.S. Pat. No. 4,805,461 to Gupta et al. and U.S. Pat. No. 4,492,121 to Lehto.
One problem with prior fluorescence systems is the small signal that is generated. To minimize the perturbation to the shock being measured, the crystal used as a sensor needs to have small dimensions. FIGS. 1A and 1B show two known methods of using ruby crystals to measure fluorescence. FIG. 1A shows a ruby disk 30 about 200 microns thick and 1 inch diameter sandwiched between two sapphires 32 each about half an inch thick. Fluorescence from the ruby disk 30 is remotely measured through relay lenses 34 that deliver the exciting beam and the fluorescence between the optical fiber 36 and the ruby 30. The shock wave incident on the whole sapphire piece is measured but only the fluorescence from converging rays 38 is collected.
A minimal perturbation to the shock being measured is accorded by the configuration shown in FIG. 1B. In FIG. 1B, the ruby 40 is directly mounted onto the fiber 36. The ruby 40 is a disk about 200 microns thick and 400 micron diameter.
To resolve spatial and temporal properties of a shock wave, a smaller sensor is required; however the fluorescence signal will decrease correspondingly. Recently, these limitations of the current methods to collect signal were recognized and sophisticated nonlinear methods have been proposed to increase signal. The added sophistication, however, makes the measurements more difficult to apply to field conditions where the need is greatest. There is a strong need to collect fluorescence from small samples at fast times for single-event experiments.
The invention includes an apparatus for measuring pressure in a medium, comprising a laser for emitting light; a dichroic beam splitter that reflects the light from the laser; a first lens that receives and focuses the light from the beam splitter; a first optical fiber for receiving the light from the first lens; a crystal having fluorescence properties and having a hemispherical shape, the crystal being attached to an end of the first optical fiber; a second lens that receives and focuses fluorescence generated by the crystal and reflected by the dichroic beam splitter; a second optical fiber for receiving the fluorescence from the second lens; a spectrometer that receives the fluorescence from the second optical fiber; a streak camera connected to the spectrometer, a charge-coupled device connected to the spectrometer; and a delay generator connected to the charge-coupled device, the streak camera and the laser.
The invention will be better understood, and further objects, features, and advantages thereof will become more apparent from the following description of the preferred embodiments, taken in conjunction with the accompanying drawings.